Liquid crystal materials are widely used in making liquid crystal displays for high content information flat panel devices, specifically for personal computers, projectors and televisions. Such devices usually require relatively low power and have a satisfactory response time, and are relatively economical. The property of liquid crystals enabling use, for example, in visual displays, is the ability of liquid crystals to transmit light on one hand, and to scatter light and/or absorb light, on the other, depending upon the alignment (or lack thereof) of the liquid crystal structure with respect to a prescribed input, such as an electrical field.
The conventional liquid crystal displays including twisted nematic, supertwisted nematic, thin film transistor twisted nematic and ferroelectric liquid crystal displays are prepared by filling liquid crystal materials directly into the liquid crystal display cells. These devices operate on the principal of light polarization. Due to the method in which the devices operate they suffer from reduced optical efficiency and require backlighting in order to achieve a good brightness on the display.
Currently there are three categories of liquid crystal materials, namely cholesteric, nematic and smectic types. The invention of the present application relates in the preferred embodiment described below to use of either a nematic, cholesteric, smectic A, or ferroelectric (chiral smectic C*) liquid crystal material or to a combination of liquid crystal types. The various characteristic of the cholesteric, nematic and smectic types of liquid crystal material are described in the prior art.
For many years, a class of new liquid crystal materials has been manufactured by treating such materials with a polymeric material to form a polymer dispersed liquid crystal for use in the manufacture of displays or other devices. U.S. Pat. No. 3,872,050, issued to Benton, relates to a polyurethane/liquid crystal dispersion system, in which cholesteric liquid crystal is dispersed as discrete naked aggregates in a polyurethane film. U.S. Pat. No. 4,101,207, issued to Taylor, relates to a liquid crystal/polymer film which is formed by: 1) casting a polymer solution containing liquid crystal, or 2) mixing liquid crystal with a polymerizable monomer or prepolymer followed by polymerization. More recently, these technologies have been improved further by the use of more suitable liquid crystal and polymeric binder materials to fabricate the electronic displays and light shutters, for example. These technologies basically include three distinct methods, an emulsion method, a phase separation method and a microencapsulation method.
An example of an emulsion method is disclosed in U.S. Pat. No. 4,435,047, issued to Fergason. The encapsulated liquid crystal droplets, which are dispersed in a polyvinyl alcohol film, are opaque in the off state. However, they become transparent when an electric field is applied. The liquid crystal/polymer film called nematic curvilinear aligned phase (NCAP) film is prepared by emulsifying liquid crystal material, generally a nematic type liquid crystal material, in aqueous polyvinyl alcohol. The emulsion is drawn down on an indium-tin oxide coated substrate, which is then laminated by another indium-tin oxide film after drying. This liquid crystal/polymer film based on a light scattering does not require the polarizers to function. However, this manufacturing process permits contamination by the impurities which are contained in the process' water and dispersing agents (e.g. polyvinyl alcohol or latex) to be transmitted to the liquid crystal material. In the Drzaic article, Journal of Applied Physics, Volume 60, NO. 6, Sep. 15, 1986, at pages 2142-2148, it is reported that an aqueous based NCAP system is an interpenetrating network of liquid crystal in a polymer matrix, rather than an encapsulated liquid crystal. Furthermore, U.S. Pat. No. 4,707,080, issued to Fergason, relates to a plurality of liquid crystal volumes which are interconnected. In addition, U.S. Pat. No. 5,216,530, issued to Pearlman et al, relates to the encapsulation of a smectic phase liquid crystal in a polymer matrix (known as NCAP). See also Drzaic et al articles, SID 90 Digest, at pages 210-213.
EP 0238 626 relates to a phase separation method utilized to obtain a light modulating material. The material is prepared from an isotropic solution of liquid crystal and prepolymer, which can be a monomer or a mixture of a monomer and an oligomer. Under an ultraviolet or an electron beam irradiation, the liquid crystal droplets segregate from the insoluble polymer matrix. The liquid crystal droplets are then located in cavities within a continuous polymer matrix layer.
WO 85/04262 relates to liquid crystal droplets which are dispersed in an epoxy resin to form a new light modulating material. The film produced also exhibits an opaque characteristics in the off state, while exhibiting a clear characteristic when an electrical field is applied.
U.S. Pat. Nos. 3,499,702 and 3,551,026 relates to the incorporation of pleochroic dyes in liquid crystal materials, in order to enhance contrast. This is accomplished by the fact that pleochroic dyes align parallel to nematic liquid crystal director and respond to an electric field in a liquid crystal like manner.
In the Drzaic et al article, SID 92 Digest, at pages 571-574, it is reported that a dichroic based liquid crystal film prepared by NCAP technology demonstrates that there are possible applications, in portable high information density displays, without the requirement of backlighting. However, the device's useful life time is dependent upon capability of finding a dichroic dye with a long period of stability and the liquid crystal material's charge holding capability. The dichroic based NCAP film suffers problems due to the fact that the dye is exposed to moisture, air and ionic impurities existing in polymeric binder. As a result, dichroic dyes suffer stability problems.
For the polymer dispersed liquid crystal (PDLC) manufactured by a phase separation method, the liquid crystal material is dissolved in polymeric resin and placed between two indium-tin oxide coated substrates and is irradiated by ultraviolet light or electron beam in order to cure the resin. The liquid crystal material used in a PDLC may be either nematic, cholesteric or ferroelectric. See Crooker et al article, SID 90 Digest, at pages 214-216; Yang et al article, SPIE Liquid Crystal Displays and Applications, Volume 1257, 1990, at pages 60-67; Molsen et al article, SID 92 Digest, at pages 773-775; and Zyryanov et al article SID 92 Digest, at pages 776-777, for example, The optical films prepared by this method suffer some drawbacks as well. For instance, one of these drawbacks is the component in the system of liquid crystal/polymer are mutually soluble and can not be completely separated during phase separation. This causes a decrease in the liquid crystal/polymer film contrast, which specifically effect the dichroic based PDLC. This is due to the fact that only dye dissolved in the liquid crystal droplets can exhibit the dichroic properties for the guest-host effect.
U.S. Pat. No. 5,585,947, relates a method for making liquid crystal composite which has an interfacial material disposed between liquid crystal and encapsulating medium.
U.S. Pat. No. 4,100,103 relates a capsule wall is formed by the condensation of melamine-formaldehyde in the presence of a negatively-charged, carboxylsubstituted, linear aliphatic hydrocarbon polyelectrolyte material dissolved in the capsule manufacturing vehicle. This method has been used to encapsulate the inner phase materials such as a mixture of dye dissolved in oily material. There is no examples describing the microencapsulation of liquid crystal for an electronic display application where the refractive index of the capsule wall should be closely matched with the liquid crystal and the binder material.
U.S. patent application Ser. No. 08/827,574 filed by Hsu, discloses the microencapsulation method for adjusting the capsule wall refractive index by an interfacial polymerization of polyurethane/polyurea or polyurea structures and there also exists a highly cross-linked structure within the polymer film. Further, the capsule wall may be manufactured so as to be transparent, in order to be more suitable for use in an electronic display application.
The liquid crystal/polymer film preparation methods described in the prior art are all relates to the formation of monodomain liquid crystal configuration within the liquid crystal droplets or microcapsules. The liquid crystal director aligns in four major configurations e.g. radiant, bipolar, axial and toroidal (concentric) according to the liquid crystal and wall surface interaction (wall surface anchoring situation). The creation of scattering from liquid crystal/polymer film is basically due to the mismatch of the refractive index between liquid crystal material (n.sub.o and n.sub.e) and polymeric binder (n.sub.p) in the off state. On the other hand, the liquid crystal/polymer film should be transparent in the on state.
In the De Baets et al article, Journal of the SID, 1/2 1993, at pages 189-193, it relates to the development of a polymer-network liquid crystal display (PNLCD) by curing a mixed solution of liquid crystal and UV-curable acrylic monomer and photoinitiator in a conventional display cell at a slightly higher temperature than the nematic-to-isotropic transition temperature of the solution. The PNLCD shows a fast switching speed and high transmission at low operating voltage,
In the Wu et al article, SID 92 Digest at pages 583-586, it is reported that a sub-micron size droplet network is formed within PDLC droplets by incorporating a PVF thermoplastic and prepolymer resin in liquid crystal. The prepared PDLC film showed extremely fast switching time at a high threshold turn-on voltage (about 70 volts).
According to the present invention, the operating voltage reduction, and the optical efficiency improvement may be achieved by formation of cross-linked web-like structure (polymer network) within liquid crystal microcapsules. The web-like structure induces the disclination of liquid crystal alignment and formation of multidomain (polydomain) liquid crystal configuration. The web-like structure is a three-dimensional polymer network randomly oriented in a liquid crystal within a micro-capsule. The liquid crystal molecules form a continuous layer within the micro-capsule.